Smooth leaves

Dr Adam Egri and colleagues from Hungary and Germany carried out both computer modelling, and tests, on real leaves. In their experiments, they used both little glass balls and water droplets.

In their first study, they covered the surface of a smooth leaf with glass balls ranging in size from 2 to 10 millimetres. They exposed them to the sun for periods between one and nine hours. And sure enough, there were scorch marks.

So it would seem as though there was some truth to the claim that water droplets could burn leaves. Mind you, glass balls keep their shape, unlike water droplets that spread out and flatten. The other difference is that glass bends light about 15 per cent better than water does.

So the second study had water droplets on smooth leaves. But, no matter how they altered various factors, they could not get the sunlight shining through the water droplets to burn the smooth leaves.

When the sun was shining from directly above, the light would be brought to a focus underneath the water droplet, and inside the body of the leaf. But there was a big blob of water sitting on top of the leaf, soaking up the heat. Any heat that got into the leaf was immediately transferred to the water. Then, as the water got warmer, it evaporated itself out of existence. And once there was no water droplet, there was no focusing of the heat. So there were no burn marks on the leaf — for the midday sun.

What about sunlight in the morning and afternoon, when the slanting light was beaming through the water droplet, and landing on the bare leaf just outside water droplet? Could that cause a burn? The answer turned out to be 'no'.

Well, when the Sun is just above the horizon, the sunlight is travelling through about 22 times more air than when the Sun is directly above you, at the vertex, around midday. So there's not enough heat to burn.

What about the shape of the water droplet? After all, different leaves have different degrees of 'hydrophilia' or 'water-loving' tendency. So the scientists tested leaves from various trees — maple (very water-loving), plane (less water-loving) and rowan (water-hating, or 'hydrophobic').

At the water-loving end (maple), the water droplets were very flattened, while at the water-hating end (rowan), the droplets were fairly spherical.

As you would expect, the more spherical water droplets on the water-hating leaves did concentrate the heat better, but not enough to scorch the leaf. So sunlight shining through water droplets doesn't burn smooth leaves.

Hairy leaves

But what about the hairy plants? The leaves have hairs that are 'hydrophobic' or 'water-hating'. Under the right circumstances, these hairs could catch a water droplet and keep it suspended at the right distance above the leaf, to allow the focus point of the sunlight to land on the surface of the leaf. Because the water was not in contact with the leaf, there was no cooling effect. And the hairs, being water-hating, would repel the water, and so keep it spherical — the best shape to focus the sunlight.

But the hairs hate water, and the merest breath of wind would shake the droplets off. The smaller droplets were better at focusing the heat — but being smaller, they would also evaporate very quickly.

Even so, the scientists did sometimes see scorch marks produced by water droplets on hairy leaves.

How did this 'don't-water-plants-at-midday' story arise? Who knows?

But you'd think that after half-a-billion years of bio-evolution, plants (which have adapted to every conceivable ecological niche) would have learnt to deal with a little rain ...